A packaging laminate of the type which is described above is known from, for example, international patent application carrying publication number WO97/02140. The prior art packaging laminate has a rigid, but foldable core layer of paper or paperboard and outer, liquid-tight coatings of moisture and heat resistant plastic material on both sides of the core layer. In order to impart to the prior art packaging laminate tightness properties also against gases, in particular oxygen gas, the packaging laminate moreover displays an aluminium foil which serves as gas barrier and which is disposed between the paper or paperboard layer and the one outer plastic coating.
From the prior art packaging laminate, retortable packaging containers are produced with the aid of filling machines of the type which, from a web or from prefabricated blanks of the packaging laminate, form fill and seal finished packages in accordance with the so-called form/fill/seal principle well-known in packaging contexts.
From, for example, a flat folded tubular packaging blank of the prior art packaging laminate, retortable packaging containers are produced in that the packaging blank is first raised to an open, tubular packaging carton which is sealed at its one end by fold forming and thermosealing of continuous, foldable end panels of the packaging carton, for the formation of a substantially planar bottom closure. The packaging carton provided with a bottom is filled with the relevant contents, e.g. food, through its open end which is thereafter sealed by an additional fold forming and thermosealing of corresponding end panels of the packaging carton for the formation of a substantially planar top closure. The filled and sealed, normally parallelepipedic packaging container is thereafter ready for a heat treatment in order to impart to the packed contents or food extended shelf-life in its unopened packaging container.
A heat treatment which extends shelf-life may suitably be carried into effect in the manner and under the conditions described in international patent application carrying publication number WO98/16431 which is hereby incorporated as reference. The packaging container is, in this instance, placed in a retort and is heated therein with the aid of a first gaseous medium flowing in contact with the outer walls of the packaging container, e.g. hot steam, to a temperature generally within the range of 70-130° C. After a predetermined stay time at the selected temperature, the supply of the gaseous medium is discontinued. The packaging container is cooled with a second, circulating gaseous medium, e.g. cold air, and finally with a circulating liquid medium, e.g. cold water. The cooled, retorted packaging container is thereafter removed from the retort for storage, transport and/or other handling.
Even if a packaging container of the prior art packaging laminate withstands, in normal cases, a rough heat treatment which retorting entails, it nevertheless not seldom occurs that the aluminium foil embodied as gas barrier is, during the reforming of the packaging laminate into packaging containers, subjected to such powerful tensile stresses that it cracks in particularly exposed regions of the packaging laminate. The occurrence of cracks in the tensile-sensitive aluminium foil entails that the packaging container correspondingly loses tightness properties against gases which can thereby penetrate into the packaging container and come into contact with the packed product. The problem with undesirable penetration of gases through cracks which have occurred in the aluminium foil becomes particularly serious in those cases when the packed product is particularly sensitive to the action of gases, e.g. oxygen gas, which may very rapidly destroy a product and render it unusable.
One object of the present invention is, therefore, to obviate the above-described drawback in connection with the prior art packaging laminate.
A further object of the present invention is to provide a packaging laminate of the type described by way of introduction which may readily be reformed by fold forming and thermosealing into a retortable packaging container without the risk that the packaging container loses its desirable tightness properties against gases, in particular oxygen gas, even if the tensile-sensitive aluminium foil is subjected to crack formation and similar untightness during fold forming of the packaging laminate.
Yet a further object of the present invention is to provide a retortable packaging container produced from the packaging laminate for perishable and oxygen gas-sensitive food products.
These and other objects and advantages will be obtained according to the present invention by a packaging laminate and by a packaging container produced from the packaging laminate as described herein. Expedient embodiments of the packaging laminate according to the present invention have further been given characterizing features as described herein.